Normally, Base-Station (BS) antenna elements are fed from a central RF system, with a few powerful transmitting Power Amplifiers (PAs), duplex filters and a large complex transmission line system to distribute the power to all the different antenna elements. The power distributed to each antenna element must further be distributed in such a way so that each element will get the most suitable amplitude and phase weights so that the most optimal beams will be shaped. With optimal beam we mean the beam-forming that from a system perspective maximizes the Signal to Interference and Noise Ratios (SINRs) per user.
With massive Multiple Input Multiple Output (MIMO) systems comes the possibility to systemize the base-station antenna in a completely different way. We can discard the complicated feeding system and instead locate small active Radio Frequency (RF) circuits (like PAs and LNAs and other circuitry) very close to the antenna elements. The reason for this is that massive MIMO, by its fundamental principles is based on the fact that baseband processing involves very many base-station antenna elements, and active analogue circuits and AD/DA conversion must hence be done as close to the antenna elements as possible. For a conventional BS antenna, we have at its interface two ports for the complete array, each port normally feeding one of two orthogonal polarizations. Each port comprises one so called TX-port for a transmitter signal and one so called RX-port for a receiver signal.
For a massive MIMO BS antenna, we have at the interface a large number of ports, possibly one port for each antenna element in the complete array. One antenna element may constitute two radiators, each radiator having a different polarization compared to the other radiator. Each radiator in turn may be composed of two sub-radiators, each sub-radiator having same polarization. Each port is normally feeding one of two orthogonal polarizations. An antenna normally has two ports, one port for transmission and reception in one polarization, and another port for transmission and reception in another polarization. Between each port and its corresponding antenna element there is usually a TRX circuit including filter, mixer and amplification. It is readily understood that a MIMO antenna facilitates a higher number of antenna ports even if the actual number of antenna elements for the MIMO antenna is the same as for the conventional traditional antenna.
It is desirable to use inexpensive components in the amplifiers driving the antenna elements. One cheap technology that may be used for the components in the amplifiers is CMOS in which the breakdown voltages are typically in the order of a few volts. An example is a class-A linear power amplifier that allows 3.6V peak-to-peak voltage swing (Vpp). Class-A PA is here used as an example, and it is assumed that Vpp is simply twice the supply voltage, Vs. Special high voltage transistors, such as extended-drain MOS can be used, but that often requires additional expensive process options and also the transistor performances are often poorer. If the maximum drain voltage swing allowed by the breakdown voltages is 3.6 V, the output impedance to drive total 2 W towards the antenna would require a practically unusable output impedance of 0.8Ω for this example. Such low impedances leads to enormous currents that are extremely sensitive to resistive losses. And since practical antenna impedances are normally in the region between tenths of ohms up to hundreds of ohms, and the loss and bandwidth of an impedance Matching Network (MNW) depends on the impedance transformation ratio, the matching network would be lossy and narrow-band.